Yes, as long as they are willing and able to support the child financially and give the child the proper guidance of a parent.
It's better than having the child raised in foster care or looked after by the state.

Why not? If he/she can prove that is able to raise the child, has enough money for it, and is very serious about it, I think he/she should be "allowed" to adopt. In many societies there are a lot of children being raised by a single parent, and many end up being OK.

No. I know a lot of people here have said there should be a stipulation that they should provide proof of income or some other way to show that they can support the child, but what happens when they get laid off from their job or are injured? A two parent household can absorb this blow much better than a single parent household can. I am against single parents adopting for the same reason that I am against same sex couple adopting. A child should be given the opportunity to have both a mother and a father because each role is essential for their development. That's not to say that the child of a single parent cannot be successful, but a complete family unit is better for a child than an incomplete one. Since there is no shortage of families that seek to adopt children, I don't see why adopted children shouldn't be given the best possible option.

No.
For the same reason that in the former soviet union, homo sexuals cannot adopt.

They are not the norm that is the desired stabilising influence for a developing young human.

Just farming out unfortunate children to all comers straight, bent or single is not a responsible policy.
It is not a live stock market.

There may thousands of nice single people out there...but first step to having children, whether by birth or adoption ought to be an opposite gender partner.

All legal adoptions involve a screening process. They don't just give the child to anyone who wants him.
Most states in the USA have followup visits by a social worker to see how well the child is doing.